In conventional high-voltage DC transmission systems, in order to take measures against overcurrent accidents and the like in a transmission line, technologies have been invented that realize fast circuit breaking operation by using a semiconductor circuit breaker in which a plurality of semiconductor switches are connected in series (see Patent Document 1, for example). The problem here is power supply to gate drivers which open/close the semiconductor switches. For conventional gate-power-supply devices, a technology has been invented in which: transformer cores each wound with a secondary-side coil are provided for respective gate driver; a primary-side coil is provided so as to pass through hollow portions of the transformer cores; and high-frequency power is supplied to the primary-side coil, thereby supplying power to the gate drivers by the electromagnetic induction phenomenon (see Patent Document 2, for example).